Chemical treatments involving ion exchange cause gross onedimensional swelling in water of single crystals of vermiculite minerals. A delicate balance of forces holds the individual silicate layers (10 angstroms thick) parallel to one another although separated by several hundred angstroms. Colloidal dispersions produced from the swollen crystals are morphologically unique and show strong filmforming characteristics.
A noticeable feature of many of the soils of north-east Scotland is the presence in them of glistening yellow flakes of decomposed biotite. The light-reflecting property of this material catches the eye even though the flakes themselves are often of very small size.An analogous process to the breakdown of biotite as reported below occurs in the soil-clays of the area, and the present study was undertaken primarily in order to throw light on the mineralogical nature of the clay fractions of the soils, since the large amounts of anmrphous material associated with the soil-clay minerals renders less satisfactory the direct approach. The mineralogy of the soil-clays will be dealt with in a separate communication.
The hydrated Mg-vermiculite lattice contains double sheets of water molecules interleaved wittl silicate layers. In the fully hydrated condition, eaoh water sheet is arranged in a regular hexagonal pattern and the interlayer cations are located midway between water sheets in octahedral coordination.Release of water molecules from interlayer positions leads at first to a gradual contraction of the basal spacing from 14.81 A, during which the water network becomes increasingly distorted. The contraction ceases temporarily at 14.36 A while further water is withdrawn. When the probability that a cation has a complete octahedral "shell" of water molecules at a given instant falls below a certain value, an abrupt contraction to 13.82 A takes place during which the cations are displaced to sites near the silicate layer surfaces.Release of further water causes a contraction to a lattice (11.59 A) in which single sheets of water molecules are interleaved with the silicate layers. The 11.59 A phase is stable until only a small proportion of the interlayer water remains, when an approximately regular interstratification of 11.59 A and 9.02 A (dehydrated) layers develops. The final traces of interlayer water are retained in the interior of the crystal with great tenacity owing to the sealing of the crystal near the edges.
The literature on the complexes of n-alkylammonium ions with layer silicates is characterized by marked divergences, not only in the deductions and conclusions of the different investigators but also in their experimental results. An attempt is made to reconcile and rationalize the differences, and to form a self-consistent picture of these systems.Given a saturating solution of sufficient concentration, the complexes tend during their formation to adopt an arrangement in which silicate layers are separated by double layers of ions oriented so that the alkyl chains are extended away from the silicate surfaces at a high angle. Water molecules, salt and free alkylamine may also be present in the interlayer spaces. When the excess material is removed by washing and the complexes dried, ideally the alkyl chains from opposing surfaces move over one another and interpenetrate so as to form a single layer of ions. During the contraction, however, the chains interact with one another and tend to stick at some point short of complete interpenetration. Weak attractive forces acting to bring the silicate layers together, and consisting mainly of electrostatic forces originating in the mineral layers and van der Waals interactions arising from increasing association of opposing alkyl chains with increasing interpenetration, are insufficient to overcome this tendency fully. As a result, an alkyl chain of given length can give rise to a series of complexes differing in their XRD characteristics.Previous workers have noted the difficulty of achieving reproducible results and some have recommended grinding the complexes, but it appears that this treatment significantly alters the nature of the end-product.
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